Design Log DEC 001 3v3 enig used throughout 3v3 system removed from btb interconnect - thepinkmile/Enigma-NG GitHub Wiki

DEC-001 - 3V3_ENIG Used Throughout; 3V3_SYSTEM Removed from BtB Interconnect

  • Status: Obsolete - superseded by DEC-037
  • Date: 2025
  • Category: Electrical
  • Area: Power Module, Controller Board, Link-Alpha connector

Decision

The 3V3_SYSTEM rail (sourced from the CM5 on the Controller Board) is not routed to the Power Module over the Link-Alpha BtB connector. All 3.3V logic within the Power Module (RJ45 LED anodes, I2C pull-ups, BATT_PRES_N pull-up, reset pull-up) is powered by 3V3_ENIG, generated locally by the Power Module LDO (U7).

Rationale

  • 3V3_SYSTEM is a CM5-derived rail intended only for external peripheral interfaces (Ethernet, HDMI, USB 3.0 ports). Using it to power internal power-module logic would create a cross-domain

    dependency and complicate sequencing.

  • Generating 3V3_ENIG locally on the Power Module gives a clean, independently-controlled 3.3V supply that is always present when the Power Module is powered, regardless of CM5 boot state.

  • Removing 3V3_SYSTEM from the Link-Alpha connector freed pins 21-24, which were reallocated to extend the 5V_MAIN delivery cluster and GND return path.

Alternatives Considered

  • Route 3V3_SYSTEM from CM5 over Link-Alpha and use it for RJ45 logic. Rejected: cross-domain dependency, sequencing risk, wasted connector pins.
  • Use a second small LDO on the Controller Board to produce a local 3.3V for CM5-adjacent logic. Rejected: unnecessary complexity given 3V3_ENIG already exists.

Impact

  • Pins 21-22: Reassigned from 3V3_SYSTEM → 5V_MAIN (supplemental power pins)
  • Pins 23-24: Reassigned from 3V3_SYSTEM → GND (supplemental return path)
  • Combined 5V_MAIN capacity: 18 pins x 0.5A = 9A (was 16 pins = 8A)
  • Legacy Controller probe-access concept Pin 14: Reassigned from 3V3_SYSTEM → SW_LED_CTRL (GPIO 20) (subsequently updated; see DEC-009)